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(COMP4441)[2012](f)midterm~wslauaa^_26679.pdf
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Hong Kong University of Science and Technology
COMP4441: Computer Music

Spring 2012
Midterm Examination

Student Name: Key

Student ID:

Instructions:
1. This is a closed-book, closed-notes examination.

2. Answer all questions in the space provided.

3. Write down your name and student ID on this page.





Question
Score

1
/11

2
/10

3
/10

4
/12

5
/7

6
/20

7
/12

8
/18

Total
/100


1. Acoustics and Waveforms [11 marks]

a) How to evaluate the loudness of harmonics? And how to evaluate the steadiness of sound? (4 marks)


Choose the BEST answer for each question.
i. Harmonic number

ii. Frequency

iii. Amplitude

iv. Frequency deviation


Loudness of sound: __________ Answer: iii
Steadiness of sound: __________ Answer: iv

b) There are three vibrating strings in the diagram below, where is the period of vibration. Please give the frequency ratio f1:f2:f3 (1 being the top one and 3 the bottom one) among the three vibrating strings: (3 marks)





Frequency ratio: ______ : ______ : ______ Answer: 1.5:1:0.5

c) Suppose you are given a rubber band and someone has stretched it for you (as shown in the photo below).




Length=L


Suppose L is the length of the rubber band after being stretched, and L1 and L2 are the rubber band length stretched by Mr. Someone in two different trials, where L1>L2.
If you pluck the string with lengths L1 and L2 with the same power, you would obtain two different sounds. The frequencies for sound 1 and sound 2 as f1 and f2 respectively, and the amplitudes are a1 and a2 respectively.
Please state the relations between f1 and f2, and between a1 and a2. (4 marks)
Answer: f1>f2, a1<a2
2. Spectrum Analysis and PVan [10 marks]

Circle the BEST answer for each question.

a) When we try to analyze the spectrum of a periodic sound, we need to first isolate a single period of the sound using a window function. Which of the following is NOT one of the common window function shapes? (2 marks)

i. Rectangle

ii. Triangle

iii. Half a period of a sine wave.

iv. A period of a sine wave.


Answer: iv

b) Sounds you can produce using a musical instrument has a spectrum, which can be used as an important feature for recognizing musical instruments. What does this imply? Choose the BEST answer. (2 marks)

i. The frequency of a sound is different with respect to time, and the distribution of frequencies is unique for each musical instrument.

ii. At any time the sound you play contains a set of frequencies, and the amplitude ratio relationship within the set of frequencies is unique for each musical instrument.

iii. At any time the sound you play contains a single frequency, and the frequency is unique for each musical instrument.

iv. All the above are incorrect.


Answer: ii

c) What steps tog